The present invention relates to an apparatus for accelerating a beam of charged particles, and more particularly to a four-finger RFQ linear accelerator ("linac").
Accelerators are used to accelerate charged particles, e.g., atomic sized particles (ions), to very high velocities. At high velocities, such particles may be considered as a "beam". Such beam exhibits significant energy that can advantageously be used for research, medical, industrial or military applications.
Early accelerators were massive machines that relied primarily on the generation and control of large magnetic fields. Unfortunately, the cost and size of such accelerators limited their application to research laboratories. Further, the available beam from such magnetically controlled devices was not focussed as narrowly as needed for many applications.
In the 1970's, two Russian scientists introduced a dramatically new concept for accelerating charged particles. Instead of relying on magnetic fields, this new concept accelerated the charged particles by subjecting them to high frequency alternating electric fields, established using four poles (or a quadrupole). Because the alternating electric fields were varied at radio frequency levels, the apparatus developed for practicing this new concept became known as the radio frequency quadrupole (RFQ) linear accelerator (linac).
The RFQ linac revolutionized, and continues to revolutionize, the field of accelerator physics. Compared to the complex, massive magnetic accelerators previously used, the RFQ linac is relatively simple in construction and operation, compact, lightweight and portable. It will accept large quantities of ions with low kinetic energies and accelerate them to much higher energies. Moreover, the beam accelerated by an RFQ linac is highly focused, due to the strong quadrupole electric field focusing that is used in such a device.
Even the RFQ linac, however, has its limitations. As explained more fully below, there is a limit to the acceleration that can be achieved with an RFQ linac while still maintaining a desired narrow (focused) beam. In all RFQ linac structures, the acceleration rate is inversely proportional to the particle velocity. At some point in the process of particle acceleration, the beam focusing performance drops to the point where some change in the acceleration process is desired. Unfortunately, in the conventional RFQ linac structure, e.g., using a four-vane or four-bar configuration, there are no changes that can be made to the basic structure to rectify the inherent deterioration of the beam focusing that occurs with higher velocities.
As a result, the RFQ linac has heretofore been generally limited to use as a pre-acceleration device, e.g. coupled to an ion source and used for accelerating the ions to a first velocity and energy, e.g.,2 MeV. When higher acceleration rates and kinetic energies are needed, more traditional acceleration devices, such as a magnetically focused drift tube linac (DTL), and/or a coupled cavity linac (CCL), have had to be employed. Unfortunately, in both the DTL and CCL structures, the accelerated beam expands appreciably due to the weaker magnetic focusing, thereby making the beam more susceptible to brightness-destroying emittance growth.
Some applications require a very intense focused beam of charged particles. Charged particle beam intensity is usually measured in units of amperes. Conventional four-vane linacs have typically been able to provide a beam intensity limited to around 100 milliamperes. To increase the beam intensity, it would be desirable to double the intensity of a single beam or otherwise combine two or more beams into a single beam. This concept (of doubling or combining charged particle beams) is referred to as "funneling". Unfortunately, the basic structure of a conventional four-vane or four-bar linac does not easily lend itself to funneling.
What is clearly needed, therefore, is an enhanced RFQ linac structure, i.e., an RFQ linac that extends the range of velocities and energies available from the device, and that permits funneling, all while preserving the ruggedness, compactness, focusing and simplicity features of prior RFQ linac devices. The present invention advantageously addresses these and other needs.